This proposal is developing core synthesis ? enzymatic extension (CSEE) approach to produce large glycans and glycopeptides with most natural structural diversity. In CSEE approach, a core comprising a few sugars in the reducing end is synthesized first. Convergent core synthesis is cost-efficient with well documented methodologies. Then glycosyltransferases are used to elongate the core by following a variety of different biosynthesis pathways to generate complex and larger glycoconjugates with high diversity. Depending on high region- and stereo-selectivity of glycosyltransferases, CSEE is the most efficient approach to produce complex glycoconjugates with high fidelity. Our recent success of using 7 chemically synthesized N-glycan cores and 4 glycosyltransferases to produce 73 complex N-glycans clearly demonstrates that CSEE is an answer to the complexity and diversity of glycomes. In this program, 8 cores of O-GalNAc-Ser/Thr will be synthesized and extended with 16 glycosyltransferases to afford focused and diverse O-GalNAc-glycans/glycopeptides libraries; 3 cores of O-Man-Ser/Thr will be synthesized and extended with 9 glycosyltransferase to produce O-Man glycans/glycopeptides; O-Fuc, O-Glc and O-Xyl glycans and glycopeptides will be produced similarly. CSEE for synthesis of these O-glycan/O-glycopeptides will be fully validated in other labs. The technology advance of this proposal is to automate the enzymatic extension by using a conventional peptide synthesizer. We will fully investigate stability and optimal reaction conditions for the most synthetic useful glycosyltransferases and make the information accessible via a website. In summary, this program should make such a dream come true: scientists can have affordable access to any N- & O-glycans and glycopeptides as they have for regular peptides.